1. Field of the Invention
The present invention relates to a control unit for a ventilator controlled in response to respiratory bioelectric signal in particular the sensed diaphragmal electromyographic (EMG) activity (EAdi). It also relates to a method of controlling a ventilator controlled by a respiratory bioelectric signal.
2. Description of the Prior Art
Ventilators of the above type typically are used to support patients having insufficient breathing capacity in a hospital environment such as an intensive care unit or for simpler applications such as home care, but could also be used as an integral part of an anaesthesia machine.
A breathing effort is controlled by the breathing center in the brain, the main breathing muscle, the diaphragm, is activated through nerve transmission in the phrenic nerve. When the muscle is activated, it contracts and lowers the pressure in the thorax and thus creates an inspiratory flow.
EP 0 774 269 discloses a traditional pneumatically controlled ventilator which can choose and adapt ventilation mode to the needs of the patient. The ventilator is arranged to change from a time-controlled mode to a support mode if a respiration effort is sensed in the patient, and vice versa if the patient becomes apneic for a certain period of time. The air pressure and/or flow are monitored. This device depends exclusively on a pneumatically sensed respiratory effort, leading to positive pressure generation by the ventilator. The ventilator output is based on settings made by a physician. The device can switch only between the above mentioned ventilation modes. The ventilator output will always be based on a fixed flow or pressure setting.
The ventilator's responsiveness to the patient breathing efforts is improved by sensing the inspiratory effort by detecting the electric activity, the electromyogram (EMG), of the contracting diaphragm. The electromyographic signal detected and measured by using an esophageal catheter having an array of electrodes. The signal is conditioned and a signal representative of the diaphragmal electrical activity, the EAdi, is calculated. The supply of gas from the ventilator to the patient is then controlled in a suitable manner in proportion to the EAdi, such as the pressure delivered to the patient is controlled. The following patent documents disclose parts of this new technology.
WO 98/48877 describes a ventilator controlled by EMG signals measured by an esophagus electrode at the diaphragm. The control relates to the inspirational breathing air pressure, which is controlled proportionally to the EMG signal.
WO 99/62580 also describes a ventilator controlled in dependence of EMG signals. In this case the control relates to a closed control system based on the amplitude of the EMG signal for a given inspirational lung volume, an inspirational lung volume for a given intensity of the EMG signal, or a combination of the two.
The EMG signal from muscles associated with breathing, in particular the diaphragm, is directly related to the patient's need for breathing, which is neurologically controlled. Controlling a ventilator based on the EMG signal therefore enables a ventilation that can be directly associated with the patient's real breathing need.
A problem that may occur when controlling in dependence of EMG signals is that the signal may cease, slowly or abruptly. This can be caused by problems in capturing the signals, but can also occur because the patient does no longer generate a neurological breathing signal from the breathing centre. The latter may have different causes. One cause can be administration of respiratory depressant drugs another may be over ventilation. An abrupt loss of signal could be due to that the catheter is dislocated or disconnected by accident, such as if the patient moves in an uncontrolled way, or in a controlled way by the ventilator operator.
Another problem that may occur in EMG controlled ventilation is that the internal regulatory monitoring of inspiration and expiration phases in the ventilator is made difficult because the EMG controlled ventilator does not have the normal time based differentiation between these phases.